The hypocaust is one of the most
ancient forms of an HVAC system. Like many great innovations, it
originated with the Romans over 2000 years ago. A hypocaust is
both a primary system and a secondary system, as it creates heat and
distributes it as well.

The main use for hypocausts was
found in the large public bathhouses. Sauna rooms were created by
adding a pool of water, heated by the same fire heating the air
below. This created a hot, humid space to clean oneself and
converse with friends. The temperature could have easily reached
100 degrees Fahrenheit and the humidity could have reached 100% due
to the pools. These parameters are not exact, as the system
has not been used is many, many years.

Its purpose was to evenly heat the
room in the most efficient way possible. A hypocaust was composed
of a raised floor (typically about two feet), supported by columns
or pedestals of stone every few feet, with the space below left
open. A furnace, composed of a continuously burning fire, created
heat, which was then allowed to flow through the space below the
raised floor, thus heating the floor and rest of the room. Once
cooled, the air escaped through flues in the wall and out of vents
in the roof. The furnace takes up a fair amount of space, so
it was usually located in a separate room. The flues were
built directly into the walls so they did not take up useful space.

Connecting the aqueducts (which
brought water into the city from the mountains), water tanks and
pools were a series of tunnels. These were constructed of brick and
mortar (very crude for todayís standards, but they obviously
worked).

Like the water system, the air
flow system was connected by ducts, consisting of stone or brick
tunnels under the floor, into the large open space beneath the
raised floor, and into the wall flues. These flues also provided a
source of insulations for the room. The hot air rising created a
barrier, keeping the warmth inside the building.

As mentioned above, the typical
materials used in this system are stone, handmade brick and mortar.
An early form of concrete may also have been used, as the Roman had
used it for many other buildings of the age (for example, the dome
of the Pantheon). Due to the second floor and pillars, this was
difficult and laborious to construct. Costs were much higher than a
simple fireplace due to the larger amount of materials used.

The main disadvantage of the
hypocaust system is a very dangerous one. The fumes created by the
fire in the furnace easily crept out of the holding space below the
false floor and into the main space. This silent killer is now
known as carbon monoxide. Although it is easily detectable and
preventable today, the Romans probably had no idea of this concept.

Another disadvantage with this
system is the possibility of the fire becoming unmanageable and
getting out of control. A stone or concrete building may survive,
but the occupants may not.

Needless to say, the hypocaust is
no long used today. However, we do have a modern version of this
concept. Radiant floor heating uses the basic concept to evenly
heat the room, without the dangers of an open fire and fumes.
It is also much cheaper to do this system than a modern hypocaust.

As an aside, the
hypocaust is similar to a Korean
Ondol. It is
basically set up in the same way, with sub-floor heating providing
warmth for occupants. This is also no long used, due to the
same carbon monoxide and overheating problems.

Typical Uses

This system
had a use in ancient times, but with todayís safety codes it is
virtually impossible to use it in current time. This is due to the
fact that hypocaust systems have, in comparative terms, a high
probability that the occupant will become ill or possibly die from
carbon dioxide poisoning. However, as a last resort to help
struggling countries and villages, it might not be a bad idea to
teach them to use these systems and maybe apply modern day
synthetics or textiles to help avoid the danger (at least this is
the only way we can see a modern day application for this system).

Parameters

Temperature ranges for a hypocaust
system are not user controlled due to the fact that you canít
control the heat coming off of an open flame. For this reason there
are only two real settings for a hypocaust system; on and off. This
makes the low temperature that can be reached depend on the outside
weather. The maximum temperature that a hypocaust system can reach
is about 100oF and humidity can reach about 100%. The
humidity can be added by heating pools of water. Again there is not
much control that this method provides.